(i) Field of the Invention
The invention relates to a cable broadcasting system in which whether or not broadcasting signals should be outputted from tap output terminals of a tap device provided on a transmission line for transmitting broadcasting signals is set by command signals transmitted from a center equipment to a transmission line.
(ii) Description of Related Art
Heretofore, in a cable broadcasting system such as a CATV system where broadcasting signals such as television signals and the like are transmitted to a subscriber's terminal via a transmission line consisting of a coaxial cable and the like, tap devices for branching the broadcasting signals (the so-called tap-off) from the transmission line and transmitting the signals to the subscriber's terminal are dispersedly provided on the transmission line.
Some of such tap devices for the cable broadcasting system are known to be designed so that it is set by command signals from the center equipment whether broadcasting signals should be outputted from the tap output terminals or not.
Specifically, a relay that can transmit broadcasting signals of high frequency with a small loss is provided in the signal pass between a directional coupler by which broadcasting signals are partially branched from a transmission line and a tap output terminal for outputting the branched broadcasting signals to the subscriber's terminal. The relay is the so-called high frequency relay. Usually, a latching relay capable of keeping the condition of on or off is used. In the above-mentioned electronic device, or the tap device, it is designed that by supplying the relay with electricity in response to a command signal transmitted from the center equipment via the transmission line, the condition of on or off of the signal pass is easily changed (that is, the broadcasting signal is transmitted to the subscriber's terminal or not) from the center equipment.
In a cable broadcasting system equipped with such tap devices, a characteristic identifying data (the so-called “address”) is given to each tap device. When the center equipment sets the condition of output or stop of broadcasting signals from tap output terminals of a specific tap device, the center equipment produces a command signal by adding a command data (the so-called “command”) expressing a content of the command to the address of the corresponding tap device and transmits the command signal to the transmission line. In the tap device, command signals transmitted from the center equipment are received by a receiving circuit, a command signal having the same address as its own address is selected among the received command signals by a control circuit and the operation mode is set according to the command contained in the command signal.
Therefore, in such a kind of cable broadcasting system, for example, when output of broadcasting signals from a tap output terminal of a specific tap device to a subscriber's terminal equipment is to be stopped or started in response to the subscriber's demand, output or stop of broadcasting signals from the tap output terminal of the specific tap device can be easily set only by transmitting a command signal from the center equipment to the tap device which is the object of setting of tap output. In the case, there is no need for the operator to visit the place where the tap device is equipped for setting the tap output of each tap device.
In the above-mentioned tap device, electricity for operation must be supplied from the outside in order to operate a relay after receiving a command signal from the center equipment. In a cable broadcasting system provided with the above-mentioned tap device, the transmission line is sectioned into plural districts. A power supply device is provided in each district for supplying tap devices in each district with electricity. A power voltage for operating built-in circuits is produced from the power signal supplied to the transmission line in each tap device, by supplying power signals from the power supply device to the transmission line.
However, in the above-mentioned conventional broadcasting system, for example, setting of tap output is occasionally demanded for all or some of the tap devices on the transmission line at the center equipment side.
In such a case, the center equipment produces command signals in order for the plural tap devices for setting tap output and transmits the signals to the transmission line. Heretofore, the order of transmission of command signals was decided according to, for example, order of addresses given to tap devices or order in a list of tap devices defined by the outer equipments which demanded tap output setting. Specifically, since the order of transmission of command signals was determined unconditionally by addresses of tap devices or a list made by input from the outer equipments, transmission of command signals was occasionally concentrated to tap devices in the same district which was supplied with electricity by the same power supply device.
As a consequence of concentration of transmission of command signals to tap devices in a specific district, load on the power equipment for supplying the tap devices in the district increased temporarily, occasionally causing failure of sufficient electricity supply to the tap devices in the district.
Specifically, since a tap device supplies a relay with electricity and sets output or stop of a broadcasting signal from a tap output terminal after receiving a command signal from the center equipment and decoding the signal, it takes a long time until the tap device completes setting operation of tap output (that is, until electricity supply to the relay is completed). Therefore, when command signals are transmitted in order from the center equipment to tap devices in the same district, plural tap devices in the same district which received command signals turn on relays at the same time and load on the power supply device in the district becomes heavy, thereby failing sufficient electricity supply to the plural tap devices.
In order to solve the problem, the capacity of the power supply device in each district may be increased. But such a means is not available because big-sizing of the power supply device causes increase of manufacturing cost, with increase of total cost of the cable broadcasting system.